1. Field of the Invention
This invention relates to an improvement in preventing loosening of a stencil, and more particularly relates to a stencil which is tightly wound in the shape of a roll and is used for a printing machine such as a stencil printer, and to a stencil perforating device which can detect a feeding direction of the stencil roll loaded therein, and automatically or semi-automatically pays the leading edge of the stencil from the stencil roll.
2. Discussion of the Background
A stencil printer with a thermosensitive digital stencil perforating capability is extensively used as a simple printer. The stencil printer is operable with a thermosensitive stencil comprising a thermoplastic resin film. In the stencil printer (called "printer" hereinafter), a thermal head selectively perforates the stencil in accordance with image data. After the perforated stencil is wrapped around a print drum, ink is fed from inside the print drum. The ink oozes out to the stencil via a porous wall of the print drum. A sheet is pressed against the surface of the print drum by a press roller or the like via the stencil. Thus, the ink is transferred from the print drum to the sheet via the perforated stencil, thereby forming an image on the sheet.
The thermal head has a number of heating elements arranged in the main scanning direction corresponding to the axial direction of the print drum. The heating elements selectively generate heat under selective control of current supply, thereby perforating corresponding portions of the stencil.
Japanese Utility Model Laid-Open Publication No. Sho 63-178134, for example, teaches a printer having a stencil perforating unit and a document reading unit for reading image data from an original document. The document reading unit is arranged above the stencil perforating unit. With this arrangement, the printer is capable of reading the image data and perforating the stencil as an integral system.
When the stencil in the printer runs out, a fresh stencil roll will be replenished as follows.
First of all, the document reading unit atop the printer is slid away from its regular position, so that the entire stencil perforating unit is visible from above the printer. Usually, a stencil is wound round a core, and is in the shape of a roll in order to save an available space in the printer. The stencil roll is loaded from above the printer into a stencil roll support of the stencil perforating unit.
Subsequently, the leading edge of the stencil is pulled out from the stencil roll, and is then cut to a prescribed length or in a prescribed shape. After the cut piece of the stencil is removed from the printer, the leading edge of the stencil is paid out to a prescribed position. Thereafter, the document reading unit is returned to its regular position.
The foregoing printer has various problems left unsolved as follows. The stencil roll replacing procedure is complicated and troublesome, and requires dexterity on the part of the user, i.e. a full-time operator may be required. In order to replace the stencil roll, the document reading unit must be disposed so as to be movable from its regular position to a standby position. This kind of structure tends to increase necessary mechanical strength and weight of an area for mounting the document reading unit, as well as the number of components. This will increase the cost of machining and assembling of the printer.
Usually, a stencil in the shape of a film strip is wound in the shape of a roll, and is supplied as a stencil roll. The stencil roll pays out the stencil therefrom as it is rotated.
Although the leading edge of the stencil is sandwiched between the thermal head and a platen roller, the stencil roll itself remains rotatable. Whenever it starts or stops rotating, the stencil roll causes inertia moment because of its own weight. Thus, when it starts an operation different from the foregoing one, the stencil roll becomes unstable due to its own weight, which will slacken on the stencil.
In such a case, only the slack portion of the stencil will be paid out as the stencil roll is rotated. Thus, even when the rotation of the stencil roll is precisely controlled, there is a discrepancy between an amount of the stencil actually paid out and a reference amount of the stencil to be paid out in response to the rotation of the stencil roll. This will lead to a problem that the stencil cannot be paid out by the amount necessary for perforation thereof.
Further, the stencil may become slack when the stencil roll is loaded into a stencil roll holding unit. Thus, the stencil may be paid out in a loose state.
Sometimes, the stencil has to be paid out in a predetermined direction depending upon a support structure of the stencil roll holding unit. In such a case, the user should be careful to load the stencil roll correctly, which is rather troublesome.